In a display device, such as a liquid crystal display device, whose display panel itself does not emit light, a so-called backlight device illuminating the display panel from its back surface is generally arranged. Examples of the backlight device include a direct-lit backlight device, a side-lit backlight device and a planar light source backlight device; as display devices become thinner and lighter, the side-lit backlight devices are widely employed.
In the side-lit backlight device, its light source is arranged near the side surface of a light guide plate, and light emitted from the light source enters the light guide plate from the side surface. The light that has entered the light guide plate is emitted from its main surface as planar shaped light, and illuminates a display panel from the side of its back surface. Since there has been growing environmental awareness in recent years, instead of a cold cathode tube functioning as a linear light source, light emitting diodes (LED) functioning as a spot light source are increasingly used as the light source of the side-lit backlight device. FIG. 7 is a schematic diagram showing the attachment structure of a spot light source in a conventional side-lit backlight device using the spot light source.
As shown in FIG. 7, in the conventional backlight device 100, LEDs 101 mounted on a flexible printed circuit board (FPC) 102 are attached to a side surface portion 103a of a lower chassis 103. The lower chassis 103 functions to accommodate a reflective sheet 104, a light guide plate 105 and optical sheets 106. Since the lower chassis 103 is formed of a high thermal conductivity material, it also functions as a heat dissipation plate. The reflective sheet 104, the light guide plate 105 and the optical sheets 106 accommodated in the lower chassis 103 are held between the lower chassis 103 and an upper chassis 107.
As shown in FIG. 7, the FPC 102 is generally attached to the side surface portion 103a of the lower chassis using a double-faced adhesive (double-faced tape) 108. However, in a method of using the double-faced adhesive 108, especially when it is used under a high temperature environment for a long period of time, the FPC 102 is likely to curl up as shown in FIG. 8. FIG. 8 is a diagram illustrating a problem encountered by the conventional side-lit backlight device 100.
One reason why the FPC 102 curls up is the life of the double-faced adhesive 108; another reason given is an operation failure that occurs at the time of assembly. Specifically, the operation failure at the time of assembly may cause bubbles between the FPC 102 and the side surface portion 103a of the lower chassis or may cause foreign matter contamination. When this type of operation failure occurs, the adhesiveness of the double-faced adhesive 108 is reduced, and the FPC 102 is more likely to curl up.
One way to reduce the curling up of the FPC 102 as described above is to increase the accuracy of the operation of assembling the backlight device 100. However, the method of enhancing the operation accuracy and thereby reducing the occurrence of bubbles and the foreign matter contamination may increase the cost of producing the backlight device 100.
Conventionally, as disclosed in patent document 1, a circuit board on which LEDs are mounted is fixed to a heat dissipation plate with a screw. For this reason, this method is applied to the backlight device 100, and thus it is possible to fix the FPC 102 to the side surface portion 103a of the lower chassis (heat dissipation plate) with a screw. However, in this case, since the FPC 102 is flexible, the circuit board is warped under a high temperature environment, and thus a contact area between the FPC 102 and the heat dissipation plate 103a may be reduced. The reduction of the contact area between the FPC 102 and the heat dissipation plate 103a makes it impossible to sufficiently diffuse heat emitted from the LEDs 101, and it is therefore likely to reduce the light emission efficiency and the life of the LEDs 101.